Development of a Submillimeter Wave Criogenic Telescope
for the Russian Segment of International Space Station

1. Introduction and objectives

The Submillimetron Project is dedicated to astronomical studies at the
submillimeter and far infrared wavelengths using a cryogenic telescope
located on the Russian segment of the International Space Station (ISS).
The Astro Space Center of the Lebedev Institute of the Russian Academy
of Sciences is responsible for developing the Submillimetron Project. The
Project was selected by the Russian Academy of Sciences in June 1997 along
with seven other Russian experiments as candidates for science astronomical
payloads on the Russian segment of the International Space Station. Currently,
the Astro Space Center is carrying out a pre-Project study under contract
with the Russian Space Agency. If the Project will succeed, experiments
with the onboard telescope can be performed between the years 2001 and
2004.

The objectives of the Submillmetron Project are twofold. The primary
goal is to perform an astronomical study in submillimeter and infrared
wavelengths of the "cold" component of the matter in the Universe (dust
in the Solar System, in the Galaxy, and in extragalactic sources, etc.),
to conduct a submillimeter wave survey, to perform studies of the spectra
of astronomical sources and their variability, to conduct cosmological
studies (study of the anisotropy of the cosmic microwave background radiation
and search for Lyman-alpha line at the epoch of recombination and secondary
heating). The secondary goal is to provide a test bed to perform the technological
experiments needed to develop follow-on projects.

The Submillimetron Project intends to fill the time gap between the
IRAS, COBE and the follow-on projects SIRTF, FIRST, and PLANK. If it succeeds,
it can provide information on research targets for these projects as well
as test/resolve some technological issues needed to build these telescopes.

The uniqueness of the proposed telescope lies in the deep cooling of
the entire telescope and the even deeper cooling of the detectors to achieve
a high sensitivity in submillimeter wavelengths. Comparative sensitivity
of the flown projects (IRAS and COBE) and projects under development (SIRTF,
FIRST, PLANK) and the Sumillimetron telescope are given in figures
1
and 2.

2. Telescope concept

The telescope will be positioned on the Russian Segment of the International
Space Station (Fig.3). The telescope assembly will be
oriented in such a way as to preclude interference from the thermal radiation
of the station elements, the Sun and the Earth. The angle between the optical
axis of the telescope and the directions to these objects will be larger
than 60 degrees in all possible telescope pointing positions.

The data registration and processing block will be located in one of
the scientific modules of the station and connected with the telescope
assembly by a cable.

Figure 3. International Space Station. Top view.

CS/SMM - Cargo Ship Progress with SUBMILLIMETRON telescope in special tansport
bay;
SM - Service Module with remote manipulator; SPA - Solar Power Array;
SPP - Science-Power Platform. Large SPA on both side of US segment not
shown.
The Cryogenic Telescope parameters are:

Technical Requirements on Star Tracker (STR)

The Star Tracker will define both the position of the optical axe of the
telescope and the telescope field of view orientation to provide the correction
signals from the Telescope Pointing System (TPS).

3. Bolometric array concept

Scientific objectives connected with observation of extremely distant objects
determine main features of the instruments and requirements to the detectors.
In accordance to the main goal of the experiment - to achieve extremely
high sensitivity in spectral density of continuum emission. These features
include: wide spectral bands and simultaneous observation in all spectral
and spatial channels, maximum number of spatial elements in field of telescope,
minimum instrumental thermal emission of cryogenic optics comparable with
extraterrestrial background. Spectral region 0.3 -1.5 mm corresponds to
minimum in spectral density of this background. Corresponding requirements
are the following.

The last figure corresponds to measure of fluctuations in number of
quanta in background radiation and can be achieved only with thermal detector
(bolometer). For phase sensitive receiver (heterodine mixer) in accordance
to indefinity principle the noise temperature is restricted by a value
about hv/k. The needed sensitivity can be achieved with extremely
low-temperature (about 0.1K) bolometer [1] using Andreev
reflection effect [2].